1. Field of the Invention
The present invention relates to color processing for generating a profile for color separation, which indicates the relationship between input color values and output signal values to be output to an image forming device.
2. Description of the Related Art
Upon generation of a lookup table (LUT) for color separation, color-separated values of representative colors are manually calculated. Note that the color-separated values are equivalent to color signal values corresponding to amounts of color materials. Using a technique described in, for example, Japanese Patent Laid-Open No. 2002-033930 (patent reference 1), color-separated values of all grid points in a color space are interpolated using those of the representative colors, thus generating an LUT for color separation.
The technique of patent reference 1 performs the interpolation to obtain smooth changes of amounts of color materials, but does not consider obtaining smooth changes of reproduced colors. For this reason, linearity between an input color space such as an RGB space and an output color space such as a CMYK space cannot be maintained. When color separation processing is performed using linear interpolation such as tetrahedral interpolation, pseudo contours may be generated, and color reproduction precision may drop.
Japanese Patent Laid-Open No. 2004-320625 (patent reference 2) discloses a technique that estimates colors from amounts of color materials, and smoothes a color separation LUT so that reproduced colors of respective grid points become more uniform in a uniform color space with respect to an RGB space. However, the technique of patent reference 2 suffers the following three problems.
As the first problem, since output values of grid points are decided according to the shape of a gamut of an output device (to be referred to as an output gamut hereinafter), gray colors in highlight and shadow regions often seem different depending on the shape of the output gamut. As the second problem, color inconstancy, which causes changes of colors that one can perceive when a light source environment changes is not taken into consideration, and a color balance of low-saturation colors such as gray, tends to be disrupted. As the third problem, since the output values of grid points are smoothed to obtain uniform reproduced colors in a uniform color space such as a CIELab space, complicated color separation is required, and it is difficult to maintain continuity of changes of amounts of color materials in a low-lightness region where color mixture is heavily used and a gamut is narrow.
With the technique of patent reference 2 having the aforementioned problems, when the printer characteristics have changed, continuity of changes of amounts of color materials tends to be disrupted. As a result, in a low-lightness part including color mixtures of various color materials, the amounts of color materials do not change smoothly, and image quality deteriorations such as generation of pseudo contours and occurrence of heterogeneity of gloss that causes different colors depending on mixed states of color materials tend to occur.